Sky-Watcher 120/600 achro unable to focus (visual)

[janek]

Hello guys,

I recently bought a SKy-Watcher 120/600 achromatic refractor hoping for nice wide-field views and pinpoint stars. It turns out that even faint stars are blurry - as if there are many overlapping images of the same star, and the "overlapping" depends a lot on my eye placement at the eyepiece. Often even faint stars can look like double stars as the image gets distorted to one or the other side (again depending on the eye placement). I was expecting a little CA around brighter objects but not this blurriness with this scope. 

I have tried various things: I fixed collimation, used different diagonals, different eyepieces (both cheap and expensive ones like Nagler 31mm, Nagler 22mm, Delos 8mm...), used a baffle to reduce the aperture. 

What helps a lot is using the baffle but that reduces the aperture from 120mm to barely 55mm. Also using eyepieces with shorter focal length seems to help a little - at least the image becomes less sensitive to eye-placement. I have a theory that the problem might have something to do with the size of the exit pupil - the bigger it is the more I need to be careful where I put my eye.

I will just add that in terrestrial observing I see a nicely focused image, as far as I can tell ..., so I am not sure that this could be a fault of the telescope's optics.

Has anyone had a similar issue with an achromatic refractor? 

Edited November 30, 2021 by janek

[Peter Drew]

Is this a new or pre-owned telescope?     🙂

[janek]

It's a brand new scope 🙂 

[Franklin]

Where are you viewing from? Has the scope cooled? Are you viewing over rooftops of houses?

[janek]

The scope was cooled properly. So far I have been able to test just from the house's rooftop. But I think I can rule out that this would have to do with the scope's temperature and atmospheric conditions. I have observed with a Dobsonian before here and could tell if it was something like that. That's why I am a bit desperate 🙂 

[vlaiv]

What is your eyesight like? Do you wear glasses usually?

F/5 scope will have large exit pupil (unless you stop it down) - and it might be that you are seeing astigmatism from your eye.

Try switching eyes to see what happens.

Another explanation is that lens is out of collimation. When you say that you did collimation on the scope - how exactly did you do it. As far as I know SW 120ST does not have lens cell that can be collimated?

Have you performed a star test? It is something you should do to see what the optics is like - or at least to check collimation.

[janek]

I wear glasses but not during observation. I will give it a try to see if my other eye sees anything differently and/or try with glasses.

I used laser to collimate the scope. The focuser on this model has two screws that enable collimation to some extent. 

I tried to perform a star test and interestingly when out of focus the star appears elliptical (I did not see any concentric rings though, I was too focused on the elliptical shape). The elliptical shape is more pronounced in lower powers and less in higher powers. Would this point to astigmatism in my eye? Strangely I never noticed this in my Dobsonian.

[Franklin]

I think the objective may be tilted slightly in its cell. Because it's new I would contact the supplier for a replacement. If it was used I would loosen the retaining ring and tap gently around the cell to settle the doublet (making sure the ota is kept upright at the time) I had a similar problem with a 150 evostar, once I'd done the above it was fine.

[vlaiv]

1 minute ago, janek said:

The elliptical shape is more pronounced in lower powers and less in higher powers. Would this point to astigmatism in my eye? Strangely I never noticed this in my Dobsonian.

Actually yes.

Do you have astigmatism in your eyes? Astigmatism is usually function of pupil size. Larger exit pupil and more dilated your eye pupil - more likely that you'll see it.

At low powers, exit pupil is very large with fast scope. At higher power exit pupil shrinks down. There are cases where people with astigmatism in their eyes can do planetary observation without wearing their glasses because exit pupil is too small for astigmatism to even show up in image.

Similar thing happens during daytime use - if your scope seems sharp during daytime - it could be down to same reason. During daytime use - there is plenty of light and your eyes act as aperture stop - your pupils shrink and image is not affected by astigmatism even on low powers.

Try changing observing eye, try observing with glasses. It looks like at least some of the problem might be down to this.

As far as newtonian is concerned - there could be couple of reasons why you did not notice this before. It's probably slower scope so exit pupils are smaller. There is also central obstruction that removes central part of exit pupil so exit pupil again uses smaller section of your eye lens - less area / less distortion.

[vlaiv]

9 minutes ago, janek said:

I tried to perform a star test and interestingly when out of focus the star appears elliptical (I did not see any concentric rings though, I was too focused on the elliptical shape). The elliptical shape is more pronounced in lower powers and less in higher powers. Would this point to astigmatism in my eye? Strangely I never noticed this in my Dobsonian.

There is another possibility here - you can also check for that.

Do in/out focus star images look the same, or do they "change direction"?

It could also happen that one side of focus star image is elliptical, while on the other it is normal.

- if they stay the same - most likely cause is in your eye

- if ellipse elongation rotates by 90° - then it is down to lens adjustment

- if there is ellipse one side of focus but not the other - then it might be focuser sag / draw tube not being straight. ST120 does not have very good focuser and this can happen - you can try to fix it if there is play/sag by adjusting / tightening it up

In any case - more experimenting is needed to pin point the cause.

[janek]

If I have astigmatism that would be new to me, but can be very well possible. I will definitely try your suggestions tonight. Tightening up the focuser was indeed needed and I did that as one of the first things. 

The star images are elliptical when both in/out focus and they also rotate by 90 degrees. If this indeed is a fault in the scope, is it common to request a replacement in this kind of case?

[Franklin]

If it's brand spanking new and you have a receipt then yes. But from what you just described it sounds like astigmatism in the scope and not your eye.

[Franklin]

If you have a short cheshire collimating eyepiece, try that in the focuser with the lens cap on. The reflected doughnuts should be central. The astigmatism is caused by optics being out of alignment. The cell is non-collimatable so you will have to adjust the focuser to get it something like. Because it's such a short focal length scope the laser may well be pointing through the center of the objective but this will not show up any tilt. Either the doublet is tilted in the cell or the focuser drawtube is tilted with respect to the objective. I think it's fixable but on the other hand, why should you have to when it's new?

[Franklin]

1 hour ago, janek said:

But I think I can rule out that this would have to do with the scope's temperature and atmospheric conditions.

I have a South facing balcony which I sometimes use for quick looks. Beyond the balcony is a housing estate made up of bungalows. It's doable in the summer, late at night when the rooftops have had time to lose their heat. But in winter when everyone has the central heating on it's a no-no. With the constant heat plumes rising, it's like trying to use a telescope from the bottom of a swimming pool.

[vlaiv]

3 hours ago, janek said:

The star images are elliptical when both in/out focus and they also rotate by 90 degrees. If this indeed is a fault in the scope, is it common to request a replacement in this kind of case?

This points to a fault in scope, and yes, asking for replacement would be good idea as lens need to be adjusted but no means of doing that is provided for the end user.

[janek]

I did some more testing tonight and it turns out I have some bad astigmatism in my left eye. The right eye is a bit better. Thank you guys for helping in identifying the issue! I had no idea I had this. 

Now I need to fix my eyes, but at least I don't have to return my scope 🙂 

 

[vlaiv]

3 minutes ago, janek said:

I did some more testing tonight and it turns out I have some bad astigmatism in my left eye. The right eye is a bit better. Thank you guys for helping in identifying the issue! I had no idea I had this. 

Now I need to fix my eyes, but at least I don't have to return my scope 🙂 

 

Try observing with your glasses on. If you have cylinder mentioned in your glass prescription it should sort (at least some) astigmatism.

I have severe astigmatism in my right eye - I can't even observe at high power with it, but left is perfectly good - so I exclusively observe with my left eye.

[janek]

I tried with glasses on but unfortunately they have a lot of small deformities which became quite obvious during the observation - everything was even blurrier with them. I will get new ones soon, but the eye relief in my Nagler 31 and 22 mm is quite small and does not allow observing with glasses on anyway (I tried and could see only about half the field). I might consider getting contact lenses just for this if it bothers me too much and for now I will try to learn how to observe with my right eye.

Thank you vlaiv for the great advice, helped a lot to clear this up for me

Edited November 30, 2021 by janek

[Franklin]

I've been reading up on this and apparently there is a test you can do to check if it is your eyes 100%. 

View the slightly out of focus star image with its astigmatic oval shape and then turn your head as much as possible with respect to the eyepiece. If the oval shape moves with you then that confirms the astigmatism is in your eye. If oval stays put as you turn your head then astigmatism is in the optics of the telescope. In this case it could be in the diagonal, the eyepiece or the objective.

[janek]

Thanks for making this point, I tried that yesterday and the oval shapes "travel" along with my eye, time to have them checked by a doctor. I have read elsewhere that people have found contact lenses very helpful for observing so I might consider those as well in the future.